I know the Left Behind series is fictional, but is there any truth to the part where so many people received Christ after the rapture, or is it too late by then?

The problem with the fictional series Left Behind is that it is based on religious concepts not found in the Bible. You see, the Bible doesn't speak of a rapture, let alone people having an opportunity to become Christians after this event. What many people don't realize is that the rapture is a relatively modern idea, created back in the 1800's. There are several articles discussing this and other premillennial theories:

An Overview of Premillennial Theories
What is the Rapture?
People Love To Have It So: The Rapture
The Ruse of the Rapture

The sermon "Will There be a Rapture?" discusses LaHaye's Left Behind series and points out many of its flaws. If you have further questions after reading these articles and sermons, please let me know.

See also:

Questions and Answers regarding the Second Coming
Questions and Answers regarding Premillennialism